This invention relates to magnetic recording media, and more particularly, to magnetic recording media of metal thin film type, and a method for conducting recording/reproducing operation in such media.
Among magnetic recording media for use in video, audio and other applications, active research and development works have been made on magnetic recording media, usually magnetic tapes having a magnetic layer in the form of a continuous thin film because of the compactness of a roll of tape.
The preferred magnetic layers for such continuous metal film type media are deposited films of Co, Co-Ni, and similar systems formed by the so-called oblique incidence evaporation process in which cobalt and optional elements are evaporated in vacuum and directed at a given angle with respect to the normal to the substrate because such evaporated films exhibit superior characteristics. These media should have a flat surface because of remarkable deterioration of their properties due to a spacing loss. However, as the surface becomes flatter, the friction becomes greater adversely affecting head contact and transport movement.
Usually, the metal thin film type media have a magnetic layer as thin as 0.05 to 0.5 .mu.m so that the surface property of the media depends on the surface property of the substrate. For example, Japanese Patent Application Kokai No. 53-116115 discloses the provision of gently sloping protrusions in the form of creases or wrinkles on the substrate surface. Also, Japanese Patent Application Kokai Nos. 58-68227 and 58-100221 disclose the location of fine particles on the substrate surface, resulting in surface irregularities observable under an optical microscope with a magnifying power of 50 to 400 and actually measureable for height by means of a probe surface roughness meter. These media are, however, still insufficient in physical properties such as dynamic friction, runnability (the durability of tape which travels in frictional contact with rigid members in a recording machine), and moving stability as well as in electromagnetic properties.
Further, Japanese Patent Publication No. 39-25246 discloses the application of an organic lubricant on the surface of a ferromagentic metal thin film layer as a topcoat layer for the purpose of reducing dynamic friction. The use of lubricant has the actually undesirable problem that it tends to adhere to the associated head to eventually block the head gap.
On the other hand, the ferromagnetic metal thin film layers on substrates are formed by the oblique incidence evaporation process as a coalescence of columnar crystals which extend at an angle with respect to the normal to the major surface of the substrate and have a longitudinal diameter extending throughout the thickness of the magnetic thin layer. Cobalt and optional metals such as nickel and chromium form columnar structure particles themselves while oxygen, when added, is generally present on the surface of each columnar structure particle essentially in the form of oxides. These magnetic thin layers, however, undesirably develop a considerable difference in input and output characteristics depending on whether the media are transported forward or backward because of the shape anisotropy in the longitudinal direction of the substrate, that is, the direction of transport of the media.
The copending U.S. patent application Ser. No. 603,668 now U.S. Pat. No. 4,599,280, discloses a magnetic recording medium which can be reproduced With a reduced input-output difference irrespective of whether it is transported forward or backward. When coercive force is measured by changing the measuring direction in a plane defined by a longitudinal direction of the substrate and a direction normal to the major surface of the substrate, this magentic recording medium meets the following relationship: EQU (Hc.sub.max -Hc.sub.min)/Hc(0).ltoreq.0.9
where Hc.sub.max is the maximum coercive force, Hc.sub.min is the minimum coercive force, and Hc(0) is a coercive force in the longitudinal direction of the substrate. It is also disclosed that corrosion resistance is remarkably improved when the above-defined coercivity ratio is 0.6 or lower.
Even with the controlled coercive force distribution in the above-defined plane, this medium is still insufficient in runnability and still performance in still mode.
There is the need for an advanced technique for improving the surface properties of a metal thin film type magentic recording medium which can reduce the dynamic friction and input-output difference in both forward and backward directions and enhance the runnability without adversely affecting electromagnetic properties.